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ACE semantic structures
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<p>
The ACE [Automatic Content Extraction] program of the early and mid 2000's
defined a set of semantic structures, including entities, relations,
and events.  The entities were introduced first, then the
relations, and finally the events;  the events implemented by Jet are
from the 2005 ACE evaluation. The set of entity types and relation
types changed somewhat from year to year;  Jet ACE can handle several of the
variants.
</p>

<p>
ACE also defined an external representation of these semantic
structures, APF (ACE Program Format).  For each source document,
the ACE system generates an APF document;  the original source
document is not modified.  The APF document contains character offsets
with respect to the source document to indicate the provenance
of the information extracted.  [Note:  in counting character
offsets in the source document, XML tags are not counted
and end-of-lines are counted as a single character, regardless
of whether they are actually represented as 1 or 2 characters.]
</p>

<p>
The interal representation of an APF file is an 
<tt>AceDocument</tt> object, which contains zero or more
<tt>AceEntity</tt>, <tt>AceValue</tt>, <tt>AceTimex</tt>, <tt>AceRelation</tt>,
and <tt>AceEvent</tt> objects.  (See the Jet javadoc API
documentation for details.)
</p>

<p>
The ACE representations are presented briefly at the
<a href="http://www.itl.nist.gov/iad/894.01/tests/ace/">NIST ACE
web site</a>.  Details on all the entity, relation, and
event types are available through the
<a href="http://www.ldc.upenn.edu/Projects/ACE/">ACE
web site of the Linguistic Data Consortium</a>.
</p>

<h3>Entities</h3>

<p>
Entities are the most basic building blocks of the semantic
representation.  There are 7 types of entities:  persons,
organizations, GPEs (geo-political entities:  locations
with a government), [other] locations, facilities,
vehicles, and weapons.  Each entity (class <tt>AceEntity</tt>)
has one or more mentions (class <tt>AceEntityMention</tt>)
within the document.  Each mention is either a name,
a nominal, or a pronominal mention.
</p>

<h3>Relations</h3>

<p>
An Ace relation (class <tt>AceRelation</tt>) specifies
some relationship between two Ace entities, such as
an employment relation (employee -- employer), a location
relation, or a citizenship relation.  The specific set
of types and subtypes varied a bit from year to year.
Each relation (class <tt>AceRelation</tt>) may be
expressed one or more times in a document;  each
expression becomes an instance of a relation mention
(class <tt>AceRelationMention</tt>).  A relation
connects two entities;  a relation mention
connects two entity mentions.  For Ace, the two
entity mentions connected by a relation mention
must appear in the same sentence (this simplifies
the annotation task as well as the training
procedure for relation extractors).  Later versions
of Ace included temporal information on relations,
but this is not supported by Jet.

<h3>Events</h3>

<p>
Whereas an Ace relation expresses a purely binary relationship,
an Ace event (class <tt>AceEvent</tt>) allows for a
<i>trigger</i> and an arbitrary number of arguments, each with a
<i>role</i>.  The trigger is generally a single word, the
word which most directly describes the event (typically
a verb or nominalization).  In analogy with entities
and relations, there may be one or more mentions of the
same event in a document;  each is an event mention
(class <tt>AceEventMention</tt>).  All the arguments of a
single event mention must appear within the same sentence.
These arguments may be entity mentions, time expression
mentions, or value mentions.
</p>

<p>
A time expression is an expression representing a date or
time, following the TIMEX2 standard.  Following the pattern
for entities, relations, and events, we have <tt>AceTimex</tt>
and <tt>AceTimexMention</tt> classes, although each
<tt>AceTimex</tt> has but a single <tt>AceTimexMention</tt>.
A value is an event argument other than an entity or time
expression;  currently crimes, prison sentences,
contact information, job titles, and numeric expressions
such as percentages and monetary amounts are included.  Again,
we have the two classes <tt>AceValue</tt> and
<tt>AceValueMention</tt>, although no reference
resolution is done on Ace values, so each <tt>AceValue</tt>
has a single <tt>AceValueMention</tt>.
</pp>

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